The OpenGL Shading Language (GLSL) is the principle shading languages for OpenGL. While there are several shading languages available for use in OpenGL, GLSL is the only one that is a part of the OpenGL core.

GLSL is a C-style language. The language has undergone a number of version changes, and it shares the deprecation model of OpenGL. The current version of GLSL is 1.50.

Compilation model

GLSL is quite unique among shading languages due to its compilation model. Most shading languages use a simple, one-stage model: you give it a string representing a shader for one of the shader stages, and it gives you a shader object that you can bind to the context and render with.

GLSL uses a more complicated model based on the compilation of C programs. In C, a compilation unit, typically a single source file that may include several external header files, is compiled into an object file. A number of object files are then linked into a single program.

GLSL operates in a similar fashion. A set of source files, given as a series of strings, are compiled into a shader object. This is the analog of an object file. Unlike typical shading languages, there is no requirement that this shader object represent a full shader stage. The only limitation is that it can only compile code that is appropriate for a single, specific shader stage.

Shader objects are useless in that form, just as object files from compiled C programs are useless directly. Shader objects must be linked into a single program object. Unlike most shader languages, the program objects contain all shader stages at once. Only one program can be bound for rendering at any one time, and it is illegal for a program object to omit certain stages entirely.

Legacy Note: If the fixed-function pipeline is available, a program can omit one or more stages that the fixed-function pipeline can cover.

Terminology

According to GLSL's standard terminology, a shader is just a compiled set of strings for a particular programmable stage; it does not even need to have the complete code for that stage. A program is a fully linked program that covers multiple programmable stages.

For the sake of clarity, we will adjust this slightly. When the term shader is used, it will be synonymous with the GLSL concept of program. To refer to a GLSL shader, the term shader object will be used.

Language

GLSL is a C-style language, so it covers most of the features you would expect with such a language. Control structures (for-loops, if-else statements, etc) exist in GLSL, including the switch statement. This section will not cover the entire language in detail; the GLSL specification can handle that. This section will hit the highlights of the important differences between GLSL and C.

Version

The OpenGL Shading Language has gone though a number of revisions, some of them quite substantial. As part of the OpenGL Specification, a version of OpenGL is required to support one or more specific versions of GLSL. It may optionally support more.

To specify which version of GLSL should be used, use this directive:

#version 1.50

This would tell the compiler to compile for version 1.50, or error if that version is not available.

You should put this before any other language features.

Extensions

OpenGL Extensions apply to GLSL as well. Unlike regular OpenGL, where extensions are implicitly always there whether you use it or not, GLSL extensions must explicitly be specified in the particular shader string being compiled.

Similar to the #version directive, the user can activate specific extensions with the "#extension" directive. The syntax is as follows:

#extension extension_name : behavior

The "extension_name" can also be the string "all". This means it works for all extensions. The available behaviors are:

enable: Causes the named extension to work; if the implementation does not support the extension, it only gives a warning. Fails if used with "all".

require: Causes the named extension to work; if the implementation does not support the extension, it fails. It also fails if used with "all".

warn: Causes the named extension to work; however, using the extension will emit warnings. If used with "all", then the use of any extensions will emit warnings.

disable: Prevents the named extension from working at all. Thus, any use of it will be seen as undefined syntax and cause an error. If used with "all", then this prevents any extensions from working.

You should put these definitions before any other language features.

Basic types

C has a number of basic types. What C does not have the concept of is basic vector-types: a basic type that intrinsically store more than one value. The OpenGL Shading Language does define vector types.

The basic non-vector types are:

bool: conditional type, values may be either true or false

int: a signed integer

uint: an unsigned integer

float: a floating point number

Each of these types, including booleans, can have 2, 3, and 4-component vector equivalents. The n digit below can be 2, 3, or 4:

bvecn: a vector of booleans

ivecn: a vector of signed integers

uvecn: a vector of unsigned integers

vecn: a vector of floating-point numbers

Vector values can have the same math operators applied to them that scalar values do. These all perform the component-wise operations on each component. However, in order for these operators to work on vectors, the two vectors must have the same number of components.

You can access vectors using the following syntax:

vec4 someVec;
someVec.x + someVec.y;

This is called swizzling. You can use x, y, z, or w, referring to the first, second, third, and fourth components, respectively.

The reason it has that name swizzling is because the following syntax is entirely valid:

You can use any combination of up to 4 of the letters to create a vector (of the same basic type) of that length. So otherVec.zyy is a vec3, which is how we can initialize a vec3 value with it. Any combination of up to 4 letters is acceptable, so long as the source vector actually has those components. Attempting to access the 'w' component of a vec3 for example is a compile-time error.

Additionally, there are 3 sets of swizzle masks. You can use xyzw, rgba (for colors), or stpq (for texture coordinates). These three sets have no actual difference; they're just syntactic sugar.

Matrices

In addition to vectors, there are also matrix types. All matrix types are floating-point. Matrix types are as follows, where n and m can be the numbers 2, 3, or 4:

matn: A symmetric matrix with n columns and rows. This type is equivalent to matnxn.

Swizzling does not work with matrices. You can instead access a matrix's fields with array syntax:

mat3 theMatrix;
theMatrix[1] = vec3(3.0, 3.0, 3.0); //Sets the second column to all 3.0s
theMatrix[2][0] = 16.0; //Sets the first entry of the third column to 16.0.

Structs and arrays

Constructors

All types have constructor syntax that allows you to create values of that type. Constructors use this general syntax:

type(value, value, ...);

The type is the type you wish to create, be it scalar, vector, matrix, struct, or array.

Constructors for scalar types are special. They can

Samplers

Texture access is not as simple as reading a value from a memory address. Filtering and other processes are applied to textures, and how texture coordinates are interpreted can be part of the texture access operation. For these reason, texture access is somewhat complicated.

It starts with samplers, a special type that GLSL defines. Each sampler represents a texture that is attached to the program. Samplers have a type that defines what kind of texture can be attached to them. The following samplers types are available:

These samplers are for textures with floating-point image formats. For textures with integral image formats, you can preceed the sampler type with "i" for signed integers or "u" for unsigned integers. However, there are no integral "shadow" samplers.

Uniforms

Stage inputs and outputs

Each stage in the shader pipeline can define a number of input values and a number of output values. There is an implementation-defined maximum number of inputs and outputs for each stage.

In a very few cases, there are also certain built-in outputs or inputs that can be used. These are generated or required by fixed-functionality processes. However, most inputs and outputs are user-defined.

GLSL creates linkage between outputs from one stage and inputs to the next stage very simply. Because the entire program consists of all stages at once, the linkage is made via matching the variable names. It is illegal for the types of the two variables to be different if they have the same name. This will cause a linker error.

Inputs and outputs are defined as follows:

in vec3 position;
in vec3 normal;
out vec3 vertNormal;

The "position" and "normal" are inputs, while "vertNormal" is an output of this stage.

The term "centroid" can be used in front of inputs and outputs as well. The syntax "centroid in" means . The syntax "centroid out" means .

Interpolation

The outputs from a vertex shader (or geometry shader) are interpolated across the surface of a primitive to compute the corresponding input values for the fragment shader. There are some controls that can be applied to the outputs that allow the user to decide how this interpolation takes place.

The default is to use the interpolation mechanism specified by OpenGL. This can be overridden by placing an interpolation qualifier on the output:

flat: No interpolation at all. The fragment shader value will receive a constant value over the surface of the triangle, based on the value of the provoking vertex.

smooth: Perspective-correct interpolation.

noperspective: Pure linear interpolation.

Multisampling

Multisampling causes multiple samples from a single pixel to be processed. Exactly which samples within a pixel are process is implementation defined. This can cause the interpolated value to fall outside of the actual portion of the primitive being rasterized.

This is usually fine. Sometimes, it is important that interpolated values never fall outside of the boundaries of the primitive. This would be the case if certain values would produce undefined results.

The way to handle this is to use "centroid" before the "out" and "in" value. This forces the multisampling algorithm to ensure that the interpolated value is within the boundaries of the primitive.

This does cause performance loss to some degree, so it should only be done where you have actually seen a visual glitch.

Because of this, there is a mapping layer for the program's inputs and outputs. The vertex shader's input names are mapped to attribute indices, while the fragment shader's output names are mapped to draw buffer indices. This mapping can be created before the program is linked. If it is not, or if the mapping does not cover all of the inputs and outputs, then the linker will automatically define what indices are mapped to which unmapped input or output names. This auto-generated mapping can be queried by the user after the program is linked.